The present invention relates to a method for supporting a transportation surface, such as a vehicular bridge, a railroad trestle or an elevated walkway. More particularly, the present invention relates to a homeostatic method for supporting a transportation surface such that the supporting structure is resistant to unexpected, infrequent shocks such as might be encountered during an earthquake or similar disaster.
Conventional methods for supporting transportation surfaces frequently result in essentially rigid structures, i.e., the structures do not yield appreciably on the application of an external force. When an external force is applied to such a rigid supporting structure, a variety of tensile, compressive and bending forces may be created within the structure. If the external force is sufficiently high, the supporting structure may fail, resulting in damage to the transportation surface and the risk of harm to persons and vehicles on the transportation surface, as well as to persons and objects below the transportation surface. To reduce the risk of such occurrences, existing methods for supporting transportation surfaces frequently call for overdesign of at least some portions of these rigid supporting structures.
Methods for supporting rigid structures may include the use of devices, such as rubber bearings containing a core of lead to absorb heat, to provide some degree of seismic isolation to these structures. These isolating devices have several known disadvantages. The devices depend on the interaction of specialized materials, some of which tend to deteriorate over time, resulting in a decrease in the protective capacity or the expenses associated with periodic replacement. Known bearings also are unlikely to be capable of responding to the magnitude of the displacement associated with a severe seismic event. Bearings that lack sufficient shock-absorbing capability may exaggerate rather than minimize the effects of seismic shock.
Other known methods for supporting transportation surfaces result in flexible structures, such as conventional suspension bridges, that are capable of yielding to an external force. However, because these structures generally lack means for effectively dissipating energy, they tend to store the energy associated with application of an external force in a spring-like manner, resulting in undesirable oscillation of the supporting structure. Oscillation of structures supporting transportation surfaces may disrupt use of transportation surfaces, for example, during high wind conditions. Under more extreme conditions, oscillation of the supporting structure may result in damage to the transportation surface and the risk of harm to persons and property, as described above.
The method of the present invention uses simple construction techniques and materials, requires no special maintenance, and is capable of reacting to displacements of a large magnitude. The present invention provides a method for supporting a transportation surface on a structure whose elements are in or tending toward a relatively stable state of equilibrium. "Homeostasis" is defined as "a relatively stable state of equilibrium or a tendency toward such a state between the different but interdependent elements or groups of elements of an organism or group" (Webster's New Collegiate Dictionary, G. & C. Merriam Co., 1976). Hence the method of the present invention may be referred to as a homeostatic method.
The present invention provides a method for supporting a transportation surface that includes arranging laterally spaced apart fixed bearing members on a surface adjacent a path for a transportation surface and supporting elongated elastic members on a bearing surface of the bearing members at a distance spaced inwardly from the ends of the elastic members. Each elastic member is capable of bending in proportion to the magnitude of a load applied intermediate the ends of the elastic members. A transportation surface may be placed in association with the elastic members, each of which supports only the share of the transportation member which is acting directly above it. The method of the present invention establishes an equilibrium state between the bending elastic members and the weight of the transportation surface.
Beginning from such an equilibrium state, an additional load applied intermediate the ends of the elastic members causes the midportion of each elastic member to bend from a first equilibrium position an amount proportional to the magnitude of the additional load and assume a second, more downwardly bowed position. The ends of the elastic members slide against the bearing members a distance also proportional to the magnitude of the additional load as the midportion bows downwardly. The movement of the elastic members establishes a new equilibrium state between the bending elastic members and the weight of the transportation surface. When the additional load is removed, the midportions unbow, returning to substantially the same positions as their original equilibrium positions. The ends of the elastic members slide a corresponding distance in the opposite direction, also returning to substantially the same positions are their original equilibrium positions. The midportions of the elastic members bend and the ends of the elastic members slide in a similar manner in response to a force applied upwardly against the bottom of the elastic members or a force applied against any of the bearing supports.
The bending and sliding of the elastic members in response to changes in the load supported by the structure may perform shock and energy absorbing functions when the elastic members engage the bearing surface. The absorbed energy is dissipated primarily in the form of heat generated by the frictional contact between the elastic members and the bearing surfaces. Preferably, the elastic members engage the bearing surface during bending from an external force at a homeostatic, or critical, angle, i.e., an angle within the range of about 25 to about 50 degrees from a vertical axis of support for the structure.